1. Field of the Invention
The present invention relates to a developing unit for rendering a latent image formed on an image carrier visible. The present invention further relates to an image forming apparatus (e.g., a printer, facsimile apparatus, and copier) which includes such a developing unit and employs either an electrophotographic process or an electrostatic recording process.
2. Description of the Related Art
In an image forming apparatus (e.g., a printer, facsimile apparatus, and copier) employing either an electrophotographic process or an electrostatic recording process, a toner as a developer is deposited by a developing unit onto an electrostatic latent image formed on a photoconductor (i.e., an image carrier) for rendering the electrostatic latent image visible. The deposited toner is transferred to a sheet of copy paper and fixed thereon for image formation.
In an image forming process of the type described above, most of the toner deposited on an electrostatic latent image formed on a photoconductor is transferred to a copy sheet and used up there, while a portion of the deposited toner, i.e., residual toner remaining on the photoconductor, is collected by cleaning means. How much of the deposited toner remains on a photoconductor is susceptible to ambient conditions. For example, the efficiency of image transfer falls in a hot humid environmental situation, as a result of which greater amounts of toner become residual toner particles.
Further, a small amount of toner is deposited or adhered to some regions of the photoconductor other than the region where the electrostatic latent image is formed. Such toner, which lingers in the regions without being transferred to a copy sheet, is collected by the cleaning means.
Further, in order to perform toner concentration control, a toner image of reference toner concentration is formed on the photoconductor. Such a toner image, which is not transferred to a copy sheet, is collected by the cleaning means.
The amount of toner subject to the above-described toner collection by the cleaning means amounts up to about twenty percent of the total.
Meanwhile, in consideration of making good utilization of resources, there have been proposed mechanisms for reusing collected toner. The reuse of collected toner not only contributes to an increase in copy volume but also saves time and labor for the disposal of waste toner.
For instance, Japanese Unexamined Utility Model Publication JP-U 59-166264 (1984) discloses that collected toner is discharged by discharging means of a cleaning unit and conveyed by relay conveying means to a toner supplying hopper. On the other hand, fresh toner is conveyed to the toner supplying hopper by another conveying means different from the relay conveying means for collected toner. The collected toner and fresh toner are mixed together by mixing means. The mixture of toner is supplied to a developing unit from the toner supplying hopper. In JP-U 59-166264, the collected-toner conveying means, the fresh-toner conveying means, and the collected toner/fresh toner mixing means are all required.
For example, Japanese Unexamined Patent Publication JP-A 6-110329 (1994) discloses that collected toner is fed to a toner hopper which contains fresh toner, and the toner in the toner hopper is supplied to a developing unit by a toner supplying roller. Additionally, fresh toner is supplied to the toner hopper from a toner container. Such arrangement produces the problem that collected and fresh toners cannot be mixed with each other successfully. Whereas fresh toner held in a fresh-toner hopper is supplied to the developing unit by a fresh-toner supplying roller, collected toner held in a collected-toner hopper is supplied to the developing unit by a collected-toner supplying roller. These toner supplying rollers are required as collected-toner conveying means and as fresh-toner conveying means, respectively.
Japanese Unexamined Patent Publication JP-A 9-236978 (1997) discloses that fresh toner is supplied to a toner mixing section from a toner supplying section and in addition to the fresh toner, collected toner collected by a toner recycling mechanism is delivered to the toner mixing section. The collected toner and the fresh toner are mixed together in the toner mixing section. Thereafter, the mixture of toner is supplied to a developing unit. In JP-A 9-236978, the toner supplying unit is required, and besides, the toner mixing unit is needed.
A developing unit of the type described above and an image forming apparatus having such a developing unit however produce some problems. For example, the number of components is relatively large and the size becomes larger, which results in an increase in the manufacturing cost. Additionally, the conveyance of collected toner and fresh toner is done using separate driving means and the control thereof is performed individually. Therefore, both the driving mechanism and the control mechanism become complicated.